This invention relates to AC to DC boost power conversion.
Non-isolated AC to DC boost converters accept a bipolar voltage from an AC input source and deliver a unipolar voltage to a load 18 at a load voltage greater than the instantaneous value of the voltage delivered by the AC source. "Non-isolated" means that the output of the converter is not galvanically isolated from the converter input source. Such converters are useful, for example, where a load (e.g., an isolated DC-DC converter) must be supplied with an operating voltage which is greater in magnitude than the DC voltage which could otherwise be delivered by simply rectifying and filtering the AC source. AC to DC boost converters also find use as power factor correcting preregulators. Such preregulators are controlled to maintain their output voltage at some desired unipolar value while simultaneously forcing the waveform of the current drawn from the AC source to follow the waveform of the AC source voltage. In general, AC to DC boost converters are useful in applications in which power is to be delivered from a bipolar input voltage source to a load which requires a unipolar voltage at a voltage value which is greater than the magnitude of the input source.
As illustrated in FIG. 1A, AC to DC converters have conventionally been implemented by interposing a full-wave rectifier 12 between the AC input source 14 and the input of a DC-DC boost converter 10. The rectifier 12 transforms the bipolar voltage of the AC input source 14 into a unipolar rectified voltage, Vr, suitable for powering the DC-DC boost converter. When Vr is less than Vout, the boost converter is controlled (e.g., by controller 16, which varies the timing of the opening and closing of switching elements within the boost converter 10) to maintain Vout at some desired value above Vr. In applications in which both AC to DC conversion and power factor correction are desired, the controller 16 will simultaneously control both the converter output voltage, Vout, and the waveform of the input current, Iin, such that the waveform of the current drawn from the AC source 14 follows the waveform of the AC source voltage. Examples of power factor correcting AC to DC converters are shown in Wilkerson, et al, "Unity Power Factor Power Supply," U.S. Pat. No. 4,677,366; Williams, "AC to DC Converter With Unity Power Factor," U.S. Pat. No. 4,949,929; and in Carpenter, "Boost Power Supply Having Power Factor Correction Circuit," U.S. Pat. No. 4,437,146.
Many kinds of DC-DC boost converters are known, some examples of which are illustrated in FIGS. 2A through 2D. FIG. 2A shows a pulse width modulated ("PWM") DC-DC boost converter 20; FIGS. 2B and 2C show zero-current switching ("ZCS") DC-DC boost converters 30, 40 using discrete inductors 21, 36, 46; FIG. 2D shows a DC-DC boost converter using a coupled inductor 56. All of the converters shown are of a kind, referred to herein as "shunt boost converters," which operate from a unipolar input source and deliver a unipolar output and which include a switching element 22, 32, 42, 52 in a path which carries a shunt current, Is, back toward the input source when the switch is closed and in which power may flow from the source to the load when the switch is opened. These, and other shunt boost converter embodiments, including zero-voltage switching DC-DC boost converters, are described or illustrated in Miller, "Resonant Switching Converter," U.S. Pat. No. 4,138,715; Lee, et al, "Zero-Current Switching Quasi-Resonant Converters Operating in a Full-Wave Mode," U.S. Pat. No. 4,720,667; Lee, et al, "Zero-Voltage Switching Quasi-Resonant Converters," U.S. Pat. No. 4,720,668; Liu, et al, "Resonant Switches--A Unified Approach to Improve Performance of Switching Regulators," IEEE International Telecommunications Energy Conference, 1984 Proceedings, pp. 344-351; Tabisz, et al, "DC-to-DC Converters Using Multi-Resonant Switches," U.S. Pat. No. 4,841,220; and in Vinciarelli, "Boost Switching Power Conversion," U.S. Pat. No. 5,321,348.
All of the shunt boost converters of FIG. 2 share similar principles of operation. An inductive input element (e.g., discrete inductors 21 in FIGS. 2A, 2B and 2C or the effective inductance of coupled inductor 56 in FIG. 2D) in series with the input source, Vin, exhibits a current-sourcing characteristic relative to the converter output. A switch (e.g., switches 22, 32, 42, 52) is closed during a portion of each of a series of operating cycles, resulting in a flow of shunt current, Is, which recirculates, without loss, back toward the source and away from the converter output. This results in a pulsating output current, Io, which is filtered by an output capacitor 25. Neglecting losses in circuit elements, conservation of power and Kirchoff's current and voltage laws demand that, under steady state operating conditions, Pin=Vin*Iin=Pout=Vout*Io; Iin=Is+Io; and Vs=Vin (so that the average voltage across the input inductive element is zero), where Iin, Io, Is, Vin, Vs and Vout are values which are averaged over many converter operating cycles. Under steady state conditions, Vout will be greater than Vin, Iin will be greater than Io, and, for a given amount of power throughput, and a particular output voltage, Vout, a reduction in Vin will result in an increase in both Iin and Is.
By switching at zero current, and by limiting the rate-of-change of current which can occur in the output rectifier 24, zero-current switching DC-DC boost converters, of the kind shown in FIGS. 2B through 2D, essentially eliminate both the switching losses (associated with turning a switch on or off when current is flowing in the switch) and reverse recovery losses (associated with rapid application of reverse voltage across a forward conducting diode) which are present in the PWM converter of FIG. 2A. This not only results in improved converter operating efficiency (i.e., the fraction of the average power withdrawn from the input source which is delivered to the load), it also allows for increased converter operating frequency and a reduction in the size of the inductors and capacitors used in the converter.
In general, known zero-current switching boost DC-DC converters may be operated in a variety of modes which differ in terms of how many zero-crossings are allowed to occur between switch closure and switch turn-off (at a zero crossing). In one such mode, called the "short cycle" (or "half wave") mode, the switch is turned off at the first zero crossing following turn-on. Waveforms which illustrate the operation of a converter of the kind shown in FIG. 2C operating in the short-cycle mode, having a value of input inductor 21, L1, which is very large relative to the value of the value of inductor 46, L, so that variations in Iin during an operating cycle are essentially negligible, are shown in FIGS. 3A through 3D. As discussed in the cited references, the operating frequency of a zero-current switching shunt boost converter operating in this mode is a function of both converter loading and input voltage. Also, the switch in a short-cycle converter must withstand a negative voltage for a portion of the operating cycle following turn-off (FIG. 3A). In practice, switches which can block both positive and negative voltages are not yet generally available so, as illustrated in FIG. 5A, a diode 29 is typically placed in series with a unidirectional switch 31 to provide bidirectional blocking capability in short-cycle converters. In another operating mode, called the "long cycle" (or "full wave") mode, the switch is turned off at the second zero crossing following turn-on. Waveforms for a converter of the kind shown in FIG. 2C, under circuit conditions similar to those described above for FIG. 3, are shown in FIGS. 4A through 4D. The operating frequency of such a converter will vary with input voltage but is only weakly dependent upon converter loading. The switch in a long-cycle converter need not have bidirectional blocking capability but must be capable of bidirectional current conduction (e.g., note the current reversal in FIG. 4C). Such a switch is often embodied as a unidirectional switch 31 and a parallel diode 33, as shown in FIG. 5B. The waveforms shown in FIGS. 3 and 4 are representative of the nature of the waveforms found in any zero-current switching shunt boost converter operating in the short and long cycle modes, respectively. In either operating mode, the characteristic time scale, T, for the sinusoidal variations in the voltages and currents during each operating cycle is determined by the values of inductor 46, L, and capacitor 34, C. The values of L, C and Vo also determine the peak deviation in switch current, Ip, during an operating cycle and this sets an upper limit on the average value of Is (and hence the maximum converter power rating) since the switch current will return to zero only if Iin is less than Ip.
Rectification of bipolar voltages using PWM controlled arrangements of bridge-configured switches is also known. One such circuit, shown in FIG. 1B, is described in Enjeti, et al, "A Two Quadrant High Power Quality Rectifier," PCIM '90 U.S.A., Official Proceedings of the 21st International Power Conversion Conference, pp. 86-91. By appropriate control of the switches 2,4,6,8 in the circuit of FIG. 1B, both power factor correction and bidirectional power flow (e.g., between the AC source 14 and the load 18 and vice versa) can be achieved.
It is known to arrange multiple power converters to share predictably in delivering power to a load. Power sharing between quantized energy converters (e.g., ZCS converters) is described in Vinciarelli, "Power Booster Switching at Zero Current," U.S. Pat. No. 4,648,020, and in Vinciarelli, "Boost Switching Power Conversion," U.S. Pat. No. 5,321,348, both incorporated by reference. Paralleling of PWM converters in power sharing arrays is described in Unitrode Application Note U-129, "UC3907 Load Share IC Simplifies Parallel Power Supply Design," in the "Unitrode Product & Applications Handbook 1993-94."